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Practical use of geokosmos real-time cartography technologies based on Optech's airborne laser terrain mapping system
3.2 Some practical results
The proposed technology of real time mapping assumes carrying out the aerial survey mission for desired territory with Optech’s ALTM and other aerial survey equipment. The aerial data is to be processed with Optech’s and Geokosmos’ specialised software according the following scheme:
Logically the first is a procedure of true DTM separation from full cloud of laser points. As it was mentioned, implementation of such procedure is a sophisticated task. Solution is reached upon application of special topologic analysis algorithms classifying laser point per criteria "belong/not belong" to true ground. Such algorithms are built upon two obvious postulates:
- True ground point has minimal value of geodetic elevation in comparing with ones in its vicinity.
- Spectrum of spatial frequencies of true ground surface has no high frequencies
Particular realisation of such algorithms is normally carried out by modelling mathematical surface, which delineates the laser point cloud from beneath. Spectral selection for the given surface is expressed by limiting the values of the first and second differentials of the surface regarding as a two-dimensional function of planar coordinates. In practice such an approach provides quite satisfactory results.
The next step is the procedure of aerial photos automatic mutual orientation:
- The special DTM processing is implemented with a goal to detect the DTM fragments for which the correlation algorithms of point matching can be applied effectively. It means that such fragments must be smooth enough and be free from vegetation, buildings, etc.
- The aim of the next operation is a transfer of selected “favorable” fragments (their mathematical representation) to the coordinate system of each aerial photo taking into account their visibility (these fragment may be shielded with a relief or on-ground objects).
- Using only selected “favorable” fragments (more accurate their projections into the photo plans) the stereo model is produced, which now is free from miscorrelation problems. Of course such a model is not full, because it is made up from the fragments. But it is not a problem within this real time mapping approach, because the created model has only auxiliary meaning contrary to the classical stereotopography method.
- After that the model produced during the previous stages is finally orientated relatively laser-derived DTM, that corresponds with its orientation in geodetic space. Naturally, the orientation of each frame is implemented also.
Fig. 4 The semantic power line model within geokosmos’ specialised software module ALTEXIS.
The presence of GPS principal point coordinates for each photos is strictly determines the stereo pair position in space with only one degree of freedom - angle of turning around survey basis vector. So the final true stereo model orientation relatively to DTM may be done by minimisation of spatial misalignment function for both laser-derived and photogrammetrical surfaces of terrain. This may be done with R.M.S. method, for example.
The described method is just a technological basis for the Optech and Geokosmos activity all over the world for miscellaneous application. Besides the topography survey and DTM making which are definitely the main kinds of companies activity, there are some other important application where this technology is successfully used. Among them are:
- Power lines inspection;
- GIS and land use systems;
- Forestry;
- Coastal mapping and monitoring;
- Avalanche and flood prediction;
Much more information on this issue can be provided from the articles presented in the end of this paper and also from the Internet sites of both companies.
Conclusion
The technology of real time mapping was originally introduced by Optech Inc., and Geokosmos company and successfully tested in different aerial survey projects all over the world. This technology is commercially available for customers and based on Optech’s and Geokosmos own software and hardware products and is permanently under reconstruction to enhance its effectiveness. The technology proved its applicability for wide range of topography and non-topography application where it can be used instead of combinating with classical photogrammetrical and geodetic methods.
References
- E. Medvedev, "Digital automatic orthophoto production with laser locator and aerial photography data", ISPRS WG VI/3 and IV/3 Workshop, Ljubljana, Slovenia, February 2000.
- E. Medvedev, "Automatic calibration procedure for laser scanning systems", In Proc. 5th International Airborne Remote Sensing Conference and Exhibition, San-Francisco, CA, U.S.A., September 2001.
- E. Medvedev, "Combining laser scanner and digital photography data for automation of orthophotomap production", In Proc. 5th International Airborne Remote Sensing Conference and Exhibition, San-Francisco, CA, U.S.A., September 2001.
- E.Medvedev, E.Kapralova, Danilin I., "Laser locator methods for forest structure analyses", In Proc. 3rd International Conference on geospatial information in agriculture and forestry, Denver, CO, U.S.A., November 2001.
- E. Medvedev, "Simultaneous recording of LiDAR and aerial imagery", GIM international, January 2002, #1, Volume 16, p.12.
- I. Danilin, E. Medvedev, T. Sweda, "Use of Airborne Laser Terrain Mapping System for Forest Inventory in Siberia", Proceedings of the 1st International precision forestry cooperative symposium, Seattle, WA.,U.S.A., p.67.
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